Separation of hydrocarbons



June 6, 1961 Filed Nov. 19, 1956 DETECTOR RESPONSE, MV.

DETECTOR RESPONSE, MV.

2 Sheets-Sheet 1 ISO-OCTANE 4 HEPTANE I OCTANE 2 NONANE l l 'n PARAFFINSo 5 l0 I5 TIME, MINUTES FIG.I 0 TO 0 n-PARAFFINS IN Iso-ocTANE, 450 0NON-NORMALS l I 2 T :NORMAL IPARAFFINS I o I I I o 5 l0 I5 TIME, MINUTESFIG. 3 COMMERCIAL REFQRMATE, 450 0 INVENTORZ FRANK T. EGGERTSEN BY" 4 WHIS AGENT June 6, 1961 Filed Nov. 19, 1956 TEMPERATURE F. T. EGGERTSENSEPARATION OF HYDROCARBONS 2 Sheets-Sheet 2 INVENTOR I FRANK T.EGGERTSEN HIS AGENT DETECTOR RESPONSE,

TIME, MINUTES SEPARATION OF C5 TO C n-PARAFFINS FIG. 2

United States Patent 2,987,471 SEPARATION OF HYDROCARBONS Frank T.Eggertsen, Orinda, Calif., assignor to Shell Oil Company, a corporationof Delaware Filed Nov. 19, 1956, Ser. No. 622,893 1 Claim. (Cl. 208-310)This invention relates to a process for the separation of normalhydrocarbons from non-straight-chain hydrocarbons. It relates moreparticularly to the separation of vapor mixtures of normally liquidhydrocarbons which are substantially thermally stable at a temperatureof at least about 380 C.

Various methods have been proposed heretofore to separate aromatichydrocarbons from non-aromatic hydrocarbons, such as liquid-liquidsolvent extraction, extractive distillation, azeotropic distillation andselective adsorption. These methods have been developed to commercialscale operations. However, except in special cases, equally satisfactoryprocesses for the separation of straight-chain hydrocarbons fromnon-straight-chain hydrocarbons have not been made available.

Normal parafiins are frequently undesired components in hydrocarbonfuels and lubricants: they depress the octane number of gasoline, raisethe pour point of lubricating oil and raise the freezing point of dieseland jet fuels.

It has been proposed to utilize certain natural and synthetic zeoliteshaving rigid three-dimensional anionic networks and having interstitialchannels sufiiciently large to accommodate straight-chain hydrocarbonsbut sufficiently small to exclude the branched-chain and/ or cyclichydrocarbons, to separate the straight-chain hydrocarbons from otherhydrocarbons (Barrer, US. 2,306,610). Although the normal hydrocarbonsare selectively sorbed by such solid substances, no commerciallyfeasible process has been developed to utilize this separation eflfect.

A principal object of the present invention is to provide an improvedprocess for the separation of normal hydrocarbons, saturated orunsaturated, and particularly normal paraflins, from branched-chainand/or cyclic hydrocarbons. A more specific object is to provide aprocess for the separation of straight-chain hydrocarbons containingfrom four to about twenty carbon atoms per molecule fromnon-straight-chain hydrocarbons. Still more specifically, it is anobject of the invention to provide an improved process for theseparation of normally liquid normal hydrocarbons from similarly boilingnon-straightchain hydrocarbons. It is a further specific object toprovide a process in which normal paratfin fractions of different carbonnumbers are separately recovered from admixtures with otherhydrocarbons. These objects will be better understood and others willappear to those skilled in this art from the more detailed descriptionof the invention, which will be made with reference in part to theaccompanying drawing, wherein:

FIG. 1 is a graph showing the separation at 450 C. of C to C normalparaflins from 2,2,4-trimethylpentane, the so-called isooctane ofcommerce.

FIG. 2 is a graph showing the separation of a mixture of normalparaffins.

FIG. 3 shows the separation at 450 C. of normal paraffins from othercomponents in a commercial naphtha reformate.

Zeolites having rigid three-dimensional anionic networks and havingintracrystalline interstitial channels whose narrowest cross-section hasessentially a uniform diameter, e.g., about 4 or about 5 Angstrom units,are well known to the art. They are commonly designated molecularsieves. The intracrystalline channels are generally designated pores.Such zeolites are described,

for example, in a paper entitled Zeolites as Absorbents and MolecularSieves, by R. M. Barrer, in Annual Reports on the Progress of Chemistryfor 1944, vol. 61, pp. 31-46, London (1945). More recently certainsynthetic molecular sieves have become commercially available from LindeAir Products Company. One such molecular sieve is designated MS4A. It isa zeolite of average composition 096510.04 Na OtLOO A l O .1.92: 0.09SiO plus an amount of water depending on the degree of dehydration; thecrystals are cubic, with unit cells measuring, on an edge, approximately12.26 Angstrom units, and are characterized by an essentially uniformpore diameter of about 4 Angstrom units. Another available sieve isdesignated MSSA. This is made from MS4A by replacement of approximatelyof the sodium ions with calcium ions by ion exchange. These are alsocubic crystals, having the same unit cell dimensions as MS4A, and arecharacterized by an essentially uniform pore diameter of about 5Angstrom units.

Zeolites become active for selective sorption by a treatment designed todrive oif the water originally present in .the interstitial spaces. Thespaces vacated remain and become available for the sorption of compoundsof appropriate maximum critical molecular cross-section. The zeolitesmay be subjected to temperatures of 600 C. and, in some cases up to 800C. or more without destruction of their crystalline structure. In somecases, repeated contact with steam can be tolerated withoutsubstantially affecting their structure.

Activated zeolites are generally soft, friable materials. Although theymay be used as sorbents in pure form, if carefully handled, they can bemade up in the form of particles held in shape by the addition of ten totwenty percent of an inert binder material such as clay. They may alsobe used admixed with other solids which are stable at the conditions tobe used in the process and which are, preferably, non-adsorbents orrelatively nonselective adsorbents so as not to interfere in the desiredseparations.

The basis for the separation of the normal hydrocarbons from hydrocarbonmixtures containing them and nonstraight-chain hydrocarbons which iseffected by these natural or synthetic zeolites appears to be that thenormal hydrocarbons are capable of passing into and through theinterstitial openings (pores) of the sorbent, whereas thenon-straight-chain hydrocarbons have too large a maximum cross-sectionaldiameter to do so. This is supported by the results and the followingapproximate largest cross-sectional dimensions perpendicular to thelongitudi nal axes, sometimes designated critical cross-sections, asdetermined by the use of Fischer-Hirschfelder scale models, reputed togive fair estimates of molecular size, of some representativehydrocarbons: n-paraflins, 4.9 Angstroms; monomethylparaffins, 6.3Angstroms; gemdimethylparafiins, 6.7 Angstroms; ethylparafiins, 7.2Angstroms; cyclohexane, 6.6 Angstroms; and benzene, 6.9 Angstroms. Thesedifierences in critical cross-section permit the separation of normalhydrocarbons from branched and cyclic hydrocarbons by sorption inmolecu- .lar sieves whose pore diameters are sufi'iciently large toadmit the normal hydrocarbons but not large enough to admit the othertypes. Thus, chabazite, gmelinite and a synthetic zeolite of formulaBaAl Si O ,nI-I O have been reported to have diameters of the narrowestpore crosssections between 4.89 and 5.58 Angstrom units. These sieves,as well as Linde MSSA, occlude normal hydrocarbons and do not occludenon-normals; hence they permit the described separations to be made.Mordenite has been reported to have a corresponding pore diameterbetween 4.0 and 4.89 Angstrom It, as well as Linde MS4A, ,does, notocclude even normal hydrocarbons thereafter.

3 having four or more carbon atoms. It does sorb methane and ethane,with which the present invention is not concerned.

It should be understood that pore diameters herein referred to, whichare determined by physical measurements such as X-ray methods or by thesorptive characteristics of the zeolites, may not be precisely accuratenumbers. Also, it has been found that normal hydrocarbon molecules mayenter pores whose maximum diameters are believed to be at least slightlysmaller than the apparent maximum critical cross-section diameter of themolecule. For purposes of this invention, reference to zeolites havingsubstantially uniform intracrystalline interstitial channels (or'pores)of from about 5 to about 6 Angstrom units diameter includes thoseabove-mentioned zeolites and others which have the characteristic ofselectively sorbing normal hydrocarbons of four or more carbon atoms permolecule and not sorbing non-normals, by virtue of their crystalstructure. These sorbents may also be referred to herein as zeoliticmolecular sieves of from about 5 to about 6 Angstrom unit maximum porediameter.

The necessity for substantial uniformity of the maximum pore diametersis demonstrated by the fact that when a silica gel having the sameaverage pore diameter as the molecular sieve, but a distribution ofsizes including substantial proportions both of smaller and largerpores, is used in the place of the molecular sieve, a suitableseparation is not obtained.

While the ability of certain molecular sieves to sorb straight-chainhydrocarbons from mixture with other hydrocarbons, generally in staticliquid and vapor systems and at temperatures up to about 350 C., hasbeen known, it has now been found that by operating at a temperature offrom about 380 C. to about 600 C., preferably at least 400 C. and stillmore preferably from about 450 C. to about 500 C., zeolites having rigidthree-dimensional anionic networks and having substantially uniformintracrystalline interstitial channels of from about 5 to about 6Angstrom unit diameter sorb normal hydrocarbons sufiiciently rapidly andsufliciently strongly to permit their separation from admixture withother hydrocarbons by passing the hydrocarbon mixture through a mass ofthe solid material at a relatively high space velocity in vapor phase.Although the normal paraflins are sufiiciently sorbed at this hightemperature while the other hydrocarbons pass through the solid masswith sufficient ease, the sorbed normal hydrocarbons are merely delayedin their transit and can bequickly and substantially completelyseparated from the solid shortly This makes it possible to repeat thecycle of operations sufficiently often to provide for separations at arate comparable tothat of solvent extraction and the like.

Although it has been shown before that prepared zeolites ofsubstantially uniform 5 Angstroms pore diameter sorb normal paraflins ata temperature ashigh as 350 C., it is surprising to find that the normalparaflins are still sorbed sufiiciently at higher temperatures aboveabout 380 C., and substantially atmospheric pressure, especially as highas from 450 to 500 C., and yet they are displaced so readily at the sametemperature and pressure as to make their recovery from the sorbentquick and essentially complete without the necessity to heat the sorbentto a still higher temperature.

' This is demonstrated by the results shown in Table I for relativeinitial and complete emergence times, at one atmosphere pressure, ofnormal paraflins, a normal ole-' fin and representative cyclic andbranched-chain saturated and olefinic hydrocarbons through athermostated column of Linde MS-SA molecular sieve material. Helium wasused throughout the separation 'as a sweeping gas at a constant rate.Thermal conductivity was employed to detect the hydrocarbons as theyemerged from the 4 as a pure compound in a separate run. The hydrocarbonwas added to the column by diverting the flow of helium through thehydrocarbon container to push the liquid hydrocarbon charge into thecolumn. The hydrocarbon was quickly vaporized. A small amount of airassociated with it was pushed through, the column ahead of sorbedhydrocarbon. The recording peak resulting from emergence of this air wasused as the reference point to measure emergence time. This peakappeared in 0.5 minute from the time of charging. The non-normalhydrocarbons began to emerge from the column with the air peak. Hence,their initial emergence time was zero.

Table I Sorbent 900 mg. MS-5A (3" x M" column). Sample ca. 5 mg.Sweeping gas 35 mL/rnin.

Temperature, C 300 400 450 500 300 400 450 500 Substance InitialEmergence Time, Substantially Complete min. Emergence Time, mm.

n-pentane 1.5 7 n-hexane 5 0.3 0.1 15 3 2 hexene- 0.3 2 n-octane 30 1.50.6 0.1 8 4 2 n-decane 15 4 0.4 12 4 n-dodecane 15 2. 5 l22-methylbutene-1- 4-methylpentene-2... cyclohexene 0.0 ca. 1 min.

o-xylene Whereas the non-straightechain hydrocarbons emergedsubstantially completely within 1 minute even at the lowest temperature,the normal hydrocarbons required a longer time depending on carbonnumber. The data show that it is feasible to sorb straight-chainhydrocarbons at a temperature in the range of 380 C. to 500 C. Byoperating at temperatures above about 400 C. the subsequent periodrequired to remove the normal hydrocarbons is materially reduced.

The data in Table I also demonstrate that the initial emergence time fornormal hydrocarbons increases rapidly with increase in the length of thechain, i.e., increase, in carbon number of the molecule at otherwiseequal conditions, although it does not vary for nonstraight-chainhydrocarbons. Therefore, practical considerations for large scaleapplication require that the conditions of operation, such astemperature, ratio of solid-to-hydrocarbon, length of column and rate offlow of sweeping gas are correlated to give an initial emergence timefor the lightest normal hydrocarbon to be separated sufiiciently greaterthan the essentially complete emergence time of the non-straightechainhydrocarbons that satisfactory separation between them is obtained andyet the heaviest normal hydrocarbon is sufficiently emerged (eluted)within a reasonably short time thereafter. The time for emergence of themajor portion of the heaviest normal hydrocarbon should not be more thanabout 200 times that of the lightest one to be separated, better-stillno more than times as much, and preferably no more than about 50 timesas much.

' The temperature may be raised to reduce the emergence or elution time,but this has the disadvantage of greater danger of thermal degradationof the hydrocarbon. Therefore, it is preferred to minimize this elutiontime differential by providing mixtures to be separated which haverelatively narrow boiling ranges.

The boiling points at atmospheric pressure of normal parafiins, frombutane through eicosane, are listed in Table II, as well as thedifference in boiling point between each parafiin and the next higherone (A 0.).

The-carbon number isthenumber of carbon atoms per molecule.

Boiling ranges of feeds in the process of this invention should begenerally no more than 125 C. and preferably less than 100 C. Thedifference in carbon-atomsper-moleculedesignated carbon number spreadand symbolized by Anbetween the lowest boiling and highest boilingnormal hydrocarbon in the fraction should be no more than 5, andpreferably no more than 2, when rapid recovery of normal hydrocarbon isdesired. Fractions with larger carbon number spread can be charged whenit is desired to recover each of the normal hydrocarbons as a separatecut, using a larger desorption period.

To illustrate, a hydrocarbon mixture containing n-hexane and n-heptaneas the only normal paraflins and various other hydrocarbons has a carbonnumber spread of one (An=76=1) and may have any boiling range whichencompasses the boiling points of n-hexane (69 C.) and n-heptane (98 C.)but excludes essentially the boiling points of n-pentane (36 C.) andn-octane (126 C.) and preferably differs from these by at least about C.Thus, the boiling range of the mixture may range from an initial boilingpoint of from about 41 to 68 C. to an end boiling point of from about 99to about 120 C. The minimum boiling range of this feed, therefore, isabout 31 and the maximum range is about 79 C.; to be sure that thedesired normal parafiins are included and undesired ones excluded it ispreferred to have a minimum boiling range of at least about 35 C. and amaximum range of no more than about 75 C. The temperature at whichnormal paraflins present in a hydrocarbon mixture boil may be shifted byazeotroping efiects due to aromatic or other non-paraflinic hydrocarbonsin the mixture. This will result in shifting the boiling points somewhatfrom those given in Table II, but will not substantially change therange which may be used. In any event, analytical tools available today,e.g. the mass spectrometer, make it readily possible to determine whatcut points must be used to produce a feed stock having any desiredcarbon number spread in the normal hydrocarbon components.

In some cases it will be desired to separate only one or less than allof the higher boiling normal hydrocarbons of a hydrocarbon mixture fromthe other hydrocarbons. In that case the presence of lower boilingnormal hydrocarbons is not objectionable and the initial boiling pointof the mixture may be correspondingly lower to include lighter normalhydrocarbons; a substantial portion of the lighter normal hydrocarbons,which have a relatively lower emergence time than the heavier ones, maybe eluted into the product stream which is enriched innon-straight-chain hydrocarbons. Thus, the so-called cut point may bedelayed until a portion of the lower normal hydrocarbons is removed asefiiuent with the non-normal hydrocarbons. This may be particularlyadvantageous in processing gasoline fractions where the higher normalparaffins are advantageously removed but the lower normal parafiins,e.g. C and C may be desired in the product.

The separation of C to C normal paraifins (An=3) from isooctane at 450C., in a mixture containing 2.3% wt. each of the normals, is presentedgraphically in FIG. 1. isooctane emerged completely in less than twominutes, as it flashed through the sorbent mass (Linde MS-SA) withoutany observable retardation while the normal parafiins emerged soonafterwards over a period of about twelve minutes.

FIG. 2 illustrates differences in elution times for individual normalparalfins in a C -C mixture of normal paraflins. An inert gas stream waspassed through a heated column of Linde MS5A; the mixture of parafi'lnswas added to the gas stream ahead of the column of sorbent during aperiod of a few seconds. The column was gradually heated from 300 toover 500 C., as indicated by the straight line graph.

FIG. 3 shows a typical result obtained at 450 C., for the separation bya 5 Angstrom unit molecular sieve of normal paraflins in a commercialreformate which contained essentially 0; through C hydrocarbons.

The amount of sorbent for the above separations was 900 mg. and the flowrate of (helium) stripping gas 30 to 40 ml. per minute. At lower flowrates the sorbent retains normals still longer, maln'ng it easier toseparate them from non-normals which are flash-distilled from thesorbent column.

At 45 0 C. and one atmosphere pressure with the above flow rate ofstripping gas, 300 parts by weight of the MS-5A sorbent will retardabout 7 parts by weight of n-hexane or 15 parts by weight of n-decane sothat they emerge after isooctane.

In another series of experiments on a larger scale of operation and at500 C. and using in some cases nitrogen and in others methane as sweepgas, a dehexanized portion of the same commercial naphtha reformate wasseparated over Linde MS-5A molecular sieve. The hydrocarbon mixture hada boiling range of from about 75 to about 170 C. It was a mixture ofaromatic, naphthenic, isoparaflinic and normal paraflinic hydrocarbons,with the normal parafiins ranging from 7 to 10 carbon atoms permolecule. The approximate composition on a weight basis was: 9% normalparafiins, 22% other saturates and 69% aromatics; it had an F-1-3 octanenumber of 95.4. The primary purpose was to determine the maximum chargewhich could be effectively separated over a given mass of the sorbent ona once-through basis and the minimum time required for the completecycle of operations while maintaining an effective separation. Bycharging 10 g. of the preheated hydrocarbon mixture to a column of 150g. of the sorbent, maintained at 450 C., in 15 seconds, followed by a 10seconds flush with nitrogen (methane worked equally as well) to ensureremoval of non-normal hydrocarbons, then a further 30 seconds sweep withnitrogen with recovery of essentially all of the normal hydrocarbons,followed by repetition of the cycle of operations over an extendedperiod of time, the feed mixture was separated into an by volumefraction having an F-1-3 octane rating of and a 20% by volume fractioncontaining a high proportion of normal parafiins. The sweep gas was usedat the rate of 2600 s.c.f./barrel of hydrocarbons. (s.c.f. designatescubic feet at standard conditions of temperature and pressure.) Thus,the mixture was separated at the rate of 600 g./hr. using g. of sorbent,corresponding to a liquid hourly space velocity of about 3 volumes ofhydrocarbons per volume (bulk volume) of sorbent per hour. This is asurprisingly high rate when it is considered that in commercialcatalytic cracking operations the space velocity is of the order of onlyabout 0.5 v./v./hr.

In another series of operations under similar conditions but with feedinjection for 15 seconds with 0.01 s.c.f. of N product sweep with 0.035s.c.f. N in 15 seconds, n-paraifin sweep with 0.2 s.c.f. N in 60seconds, at an average temperature of 450 C. at essentially atmosphericpressure, for 50 cycles, the C- -C commercial reformateiwas' separatedinto- 84.5% product lean in n-paraffins with a; refractive index of1.4600 andanoctane number, Fl-3 of- 98.l and 14.2% of n-paraflin-richfraction having an index of refraction of 1.4256 (20/D).

Although the separations are advantageously. effected in vapor phase atabout one atmosphere pressure and at the temperatures specified, similarseparations are obtained when thevapor mixture is subjected to anelevated pressure'which may range-up-to 100 to 750 pounds per squareinchor evenhigher up to 1000' p.s.i.g., particularly in the case of thelower boiling, gasoline range, hydrocarbons. When higher pressures areapplied to the vapor mixture in contact with the sieve sorbent, thevapor concentration is higher and either more eluting gas or a longertime is required to remove the hydrocarbon from the solid. 'However,this is compensated for in part by the 'greatercapacity of the solid forthe normal hydrocarbons so that a greater amount of charge may be addedto the solid mass before effluent non-straight-chain product ismaterially contaminated with normal hydrocarbons.

In general, in order to insure vapor phase separation, the temperatureshould be chosen so that the ratio of the absolute temperature in thecontacting zone to the absolute boiling temperature of the highestboiling component of the mixture at the operating pressure is greaterthan, unity and generally it is greater than 1.1, preferably at least1.3. The temperature may be at any suitable level above 380 C., butbelow that of thermal cracking, under process conditions of flow andpressure, of the leastv stable hydrocarbon of the mixture, which islower for more complex larger molecules than for the smaller normalparaffins. Also, the thermal stability of the solid sorbent must beconsidered, although this generally is not a primary limitation on theprocess. Although the process may be practiced at temperatures as highas about 600 0., operation at this temperature would generally requirean elevated pressure for the separation of the light normalhydrocarbons, because of the reduced relative retention of the lightnormal hydrocarbons at this temperature. Consequently, it is generallypreferred to operate ata temperature of up to about 550 C. and stillmore preferably up to about 500 C.

In processing a hydrocarbon fraction in accordance with this invention,a convenient method comprises: (1) establishing a bed of molecular sievein a vessel, such as a tube or tower, which is suitably provided withheating means such as a jacket or internal coils; (2) passing feedthrough the bed while maintaining the desired bed temperature; (3)passing the efliuent, which at first contains essentially onlynon-normal hydrocarbons to a product recovery means, which may be, forexample, arcondenser or an absorber with the usual associated equipment;(4) discontinuing flow of feed to the bed either (a) after apredetermined amount of feed (calculated to contain no morenormalhydrocarbons than the sorbent mass has capacity to. sorb) has beencharged, or (b) when normal hydrocarbons to be separated first appear inthe efiluent from thesorbent bed; (5) passing sweeping gas through thebedto displace the non-normals; (6) continuing flow of sweeping gas,preferably at a higher flow rate, and passing the eflluent to a separaterecovery means to recover normal hydrocarbons; and (7) after the normalhydrocarbonshave been removed from the bed, at least to the extent towhich they are readily desorbed at the prevailing temperature andpressure, adding fresh feed to the sorbent charge and repeating thecycle. This procedure. is readily modified to permit separate recovery'of normal hydrocarbon fractions of diflerent molecular wei ghts,originally present in a relatively wide boiling feed fraction, bycontrolling the how of eflluent during 7 V the desorptionstep so that'itis switched to a different re '8 e.g. by a mass spectrometer, readilypermits such controlled recovery.

In some cases, a small amount of normal hydrocarbons may result in thesorbent at the endof the desorp-- tion step in this process. This amountmay. vary with the temperature and other conditions used in the desorption step and with the molecular weight of. the hydrocarbon.Sufficient normal hydrocarbon is desorbed so that at most only a smallamount remains, which is very tenaciously held so that it does notappear to a substantial extent in the eflluent during the step ofrecovering nonnormals.

In the normal hydrocarbon removal step ofthis invention it is essentialto have a substantial How of an inert gas to sweep out the normalhydrocarbons. 'Helium, hydrogen, nitrogen and methane have beensuccessfully used as such sweep gases. Other inert gases, i.e., gaseswhich do. not react with either the sorbent, the vessel or thereactants, are also suitable. For example, argon, flue gas. (preferablyscrubbed to remove reactive impurities), propane and other gases andvapors can be used as sweep gas- As has been shown, it is not essentialto use a sweep gas. while the hydrocarbon feed is being charged. Thefeed may be added to the heated sorbent as a liquid, to be quicklyvaporized by contact with sorbent, or it may be added as a vapor. Ifdesired, sweep gas flow may be maintained through the sorbent mass atall times, and the. feed added as liquid or as vapor to the sweep gas.lt will generally be preferred to maintain a relatively low rate ofinert gas flow or no gas flow at all during the time when feed is added,and a substantial gas flow during thetime when sorbed normalhydrocarbons are desorbed. The first sweep gas added after feed isdiscontinued serves to flush the remaining non-sorbed hydrocarbons outof the sorbent, mass. The rate of gas flow during this flushing step maydifier from that used during the sweeping step.

Referring to FIGS. 1, 2 and 3, it is readily seen that the processconditions can be selected either to separate essentially all of thenormal hydrocarbons from the nonnormal hydrocarbons, or they can beselected so that the lower normal hydrocarbons, e.g. up to n-hexane, canbe separated with the non-normal hydrocarbons and separated from thehigher normal hydrocarbons. This is particularly advantageous in theimprovement of hydrocarbon fuels, such as gasoline, kerosene, and thelike containing mixtures of hydrocarbons wherein the carbon numberspread ofthe normal hydrocarbons is as high as 4 or 5, such as aplatformate composed predominantly of C3 to'C hydrocarbons. Themixtureswhich'contain a Wider rangejof normal hydrocarbonsmay be advantageouslyde-normalize'd 'by first separating the mixture, as by distillation,into'two or more fractions of narrower ranges, and adding thesesuccessively to a column of the sorbent, the fraction of lieavijestmolecules first, in, charge amounts selected so that the heavier normalhydrocarbons, have'traversed only 'a, part of the-column when thelightest fraction is added later and so that lightest and heaviestnormal hydrocarbons reach the exit and essentially. simultaneously. Theadditionof charge and eluting gas is then discontinued and the normalhydrocarbons are then swept outiby means of a suitable sweep gas. Bythis means, the total timefor separation and recovery of the highernormal hydrocarbons in a given column is used to greatest; advantageitoobtain further separation of other normal hydrocarbons. 7

The process of the present invention is useful in the separation ofnormal hydrocarbons from mixtures containing normals. having a carbonnumber of at least 4 and generally'at least 5 It is particularlysuitable for mixtures containing normals from n-pentaneor n-hexanethrough n-decane or n-dodecane and corresponding 9 olefins, but may beused with mixtures containing normals up to n-eicosane.

The foregoing illustrative separations demonstrate the utility of thepresent invention in a large scale process for the separation ofmixtures of normal and non-normal hydrocarbons. It is particularlyuseful in upgrading gasoline hydrocarbon mixtures which contain normalparaflin hydrocarbons, whether they are straight-run fractions,thermally cracked fractions, catalytically cracked fractions orreformates. The normal parafiins in the gasoline boiling range have lowoctane values decreasing with increasing molecular weight, and theirpresence in gasoline reformates of various origins, whether fromplatinum catalyzed reforming, such as platforming, or from reformingwith other catalysts, such as molybdenum oxide/alumina catalyst, isundesirable. However, their separation from the mixture usually isdifiicult and expensive, sometimes prohibitively so. By the presentprocess, the normal paraflins are substantially completely removablefrom the gasoline boiling range fraction, or the process is readilyadaptable to the removal of substantially all of the higher normalparafiins present while leaving a substantial portion to about all ofthe lower normal paraflins present in the non-normal-enriched productstream. The process also removes the normal olefins from the thermallyand catalytically cracked products. Similarly, for the preparation oflow freezing point highly paraflinic naphtha fractions to be used as, orfor blending to, other hydrocarbon fuels, such as jet fuels and thelike, fractions of the fuel or the entire blended fuel may be treated inaccordance with the invention to remove the higher normal parafiinspresent, while removing or leaving the lower normal paratfins asdesired, to lower the freezing or pour point of the fuel.

-I claim as my invention:

A process for the separation and recovery of normal hydrocarbons andnon-normal hydrocarbons from a plurality of mixtures of suchhydrocarbons, said plurality consisting of separate and immediatelysuccessive narrow boiling fractions differing in boiling range andcontaining normal hydrocarbons of at least carbon atoms per moleculewhich comprises maintaining a fixed bed columnar particulate mass of asolid zeolitic material having a rigid three-dimensional anionic networkand 10 having substantially uniform introcrystalline interstitialchannels of from about 5 to about 6 Angstrom units diameter at acontacting temperature of from 380 C. to about 600 C.:

(1) passing the highest boiling of said fractions in vapor phase throughsaid columnar mass to selectively retard the passage of the normalhydrocarbons through the mass while the non-straight-chain hydrocarbonspass through the mass,

(2) then passing said other fractions in vapor phase through saidcolumnar mass separately and successively in order of decreasing boilingrange, the fractions being charged in proportions selected so that thesmallest and largest normal hydrocarbons begin to leave the columnarmass essentially simultaneously;

(3) discontinuing passage of said fractions to the mass before normalhydrocarbons begin to appear in substantial proportion in the eflluentstream;

(4) passing an eluting gaseous material through the mass in the samedirection as the said fractions to elute a substantial proportion of theretained hydrocarbons as a normal hydrocarbon-enriched stream; and

(5) repeating the cycle of steps (1), (2), (3) and References Cited inthe file of this patent UNITED STATES PATENTS 2,306,610 Barrer Dec. 29,1942 2,586,889 Vesterdal et al. Feb. 26, 1952 2,651,603 Martin et al.Sept. 8, 1953 2,818,137 Richmond et a1. Dec. 31,, 1957 2,818,455 Ballardet al Dec. 31, 1957 2,834,429 Kinsella et al. May 13, 1958 2,866,835Kimberlin et al. Dec. 30, 1958 2,889,893 Hess et al. June 9, 1959 OTHERREFERENCES Article by Barter et al. in Transactions of the FaradaySociety (London), vol. 40, 1944, pp. 198 and 199.

Article by Barrer in Quarterly Reviews of the Chemical Society (London),vol. III, 1949, page 302.

Chemical Engineering News, Nov. 29, 1954, vol. 32, page 4786 (article,Selective Adsorption With Zeolites).

